Reasons to this stuff

First take a catalyst to have mana to use A and take boots, after finish Rod of ages to have life, mana and Ap and to stack him. After take a Rylai to get more tanky, have more Ap and slow with spells and auto-attack with E. Then take Rabadon to have lots of damages because now you begin to be tanky enough at this time. Take Nashor's Tooth to have Attack Speed to slow more with E and so more damages, and Reduce cooldown. To finish i take a Athen'sunholy grail to finish Ap and have Magic resists and Reduce Cooldown !